This invention relates generally to electro-optical light modulators, and, more specifically, to driving circuits therefore and packaging thereof.
The optical modulation of a light beam by a fixed frequency sinusoidal waveform is useful in several applications. Primarily employed in a coherent optical system, a laser generated light beam is passed through a substantially transparent crystal material that is subjected to a large magnitude sinusoidal voltage across it. The frequency of the modulating signal is typically within a radio frequency ("r.f.") range for one class of applications, which extends essentially from about 1 megahertz ("MHz.") to 250 MHz. One type of crystal utilized in such modulators is characterized by varying its refractive index in proportion to the amount of voltage applied to it. Such a crystal alternatively changes the path length of the beam through it in accordance with the frequency of the driving voltage. Thus, a laser beam emerging from such a crystal is modulated by the frequency of the driving voltage. That light output, in the frequency domain, includes a center frequency spectrum of the incident light beam, along with side bands on either side of the center spectrum and separated from it by the frequency of the modulating voltage. Applications of such a modulated light beam include frequency stabilization of a laser and frequency modulated spectroscopy.
Typically, the source of the modulating signal is a standard signal generator which has an output of only a few volts, typically five volts. The crystal, on the other hand, requires a much higher voltage to operate. Several hundred volts are typically necessary to change the phase of the light beam though it by one-hundred eighty degrees. In order to increase the signal generator voltage to the higher level required to drive such a crystal, a separate voltage amplifier is generally utilized in commercial and scientific applications. Such an amplifier is usually physically large and rather expensive. As an active, powered device, such an amplifier needs to faithfully amplify the input signal from the signal generator without distortion.
As a replacement for such an amplifier, an approach utilizing a passive resonant driving circuit has been suggested. Such an approach is based upon the characteristic impedance of the optical crystal being substantially purely capacitive. Thus, it looks to the modulating signal driver like a capacitor. A low resistance inductor is connected in a manner and with a value that forms a circuit having a resonant frequency substantially that of the signal generator. The low voltage output of the signal generator at that resonant frequency then appears as a much higher voltage across the capacitive crystal load. The few signal generator output volts can easily be increased to several hundred volts necessary to drive such a modulating crystal. The driving circuit has its impedance matched to the output impedance of the signal generator by use of a transformer in order to avoid energy losses by reflections at that interface.
It is a primary object of the present invention to provide an improved resonant driving circuit which is more efficient and less complicated than those previously suggested.
It is another object of the present invention to provide improvements in the design of such a resonant driving circuit that makes it suitable for a commercial product by being easily manufacturable with standard available components.
It is a further object of the present invention to provide an improved optical modulator package suitable for a commercial product.